Influence of scanning density on macular choroidal volume measurement using spectral-domain optical coherence tomography

Background

To evaluate the impact of scanning density on macular choroidal volume measurement using spectral-domain optical coherence tomography (SD-OCT).

Methods

Thirty eyes of normal subjects underwent consecutive raster choroidal scanning protocols using SD-OCT in enhanced-depth imaging mode. Manual choroidal segmentation was performed using the built-in automated retinal segmentation software to obtain five analyses with different inter-scan distances, including inter-scan distances of 30 μm, 60 μm, 120 μm, 240 μm, and 480 μm. The built-in software of the device automatically generated the choroidal thickness and volume map in the similar manner as for the retinal volume map, using the standardized Early Treatment Diabetic Retinopathy Study (ETDRS) grid. For each raster scan, mean absolute difference and relative difference of mean foveal choroidal thickness (FCT), foveal choroidal volume (FCV) and total macular choroidal volume (TCV) in comparison to “true value” (i.e., 30-μm inter-scan distance) were calculated.

Results

The maximum relative differences were 10 % and 16 % for TCV and FCV respectively. For mean FCT, the maximum absolute difference was 31 μm, and maximum relative difference was 12.7 %. No statistically significant differences were found in measurements of mean foveal choroidal thickness (p?=?0.912) and volume (p?=?0.944), as well as macular choroidal volume (p?=?0.912), with varying inter-scan distance.

Conclusions

Our study shows that approximately 16 scans over the macula with a inter-scan distance of 480 μm is sufficient to provide a clinically relevant and reliable choroidal thickness/volume map. This information could be useful in the design of choroidal scanning protocols for future clinical trials.     

Calculating the predicted retinal thickness from spectral domain and time domain optical coherence tomography – comparison of different methods

Purpose

To compare the accuracy of different methods of calculating predicted central retinal thickness values in order to allow comparison between results of spectral-domain optical coherence tomography (SD-OCT) and time-domain OCT (TD-OCT) devices.

Methods

In a prospective cohort study, 100 consecutive healthy individuals without ocular disease underwent sequential scanning with SD-OCT (Spectralis OCT) and TD-OCT (Stratus OCT). A group of 60 eyes was used to generate the conversion equations, which were tested on the remaining 140 eyes. Four equations were used: 1. Mean difference between SD-OCT and TD-OCT; 2. Multiplying a ratio by the original retinal thickness; 3. Linear regression analysis using retinal thickness; and 4. Regression analysis using retinal thickness and spherical equivalent. All four methods were used to calculate predicted SD-OCT values from TD-OCT measurements, and vice versa.

Results

For all four equations, the predicted SD-OCT central retinal thickness values were similar to the actual SD-OCT, with mean difference ranging from 0.78 to 1.01 μm, and intraclass correlation coefficients >0.88. Both regression equations and mean difference showed greater accuracy, with variation between calculated and actual retinal thickness values ≤5 μm in 60 % of eyes. In contrast, the ratio method was less accurate, with 15.8 % of eyes showing differences >15 μm. Similar results were found for predicted TD-OCT values.

Conclusions

Several methods can be used to convert central retinal thickness values from SD-OCT to the predicted TD-OCT value, or vice versa, with high degrees of accuracy and reliability. These methods may allow comparison of OCT values from SD-OCT and TD-OCT devices in clinical trials and standard patient care.     

Effectiveness of averaging strategies to reduce variance in retinal nerve fibre layer thickness measurements using spectral-domain optical coherence tomography

Background

Automated detection of subtle changes in peripapillary retinal nerve fibre layer thickness (RNFLT) over time using optical coherence tomography (OCT) is limited by inherent image quality before layer segmentation, stabilization of the scan on the peripapillary retina and its precise placement on repeated scans. The present study evaluates image quality and reproducibility of spectral domain (SD)-OCT comparing different rates of automatic real-time tracking (ART).

Methods

Peripapillary RNFLT was measured in 40 healthy eyes on six different days using SD-OCT with an eye-tracking system. Image brightness of OCT with unaveraged single frame B-scans was compared to images using ART of 16 B-scans and 100 averaged frames. Short-term and day-to-day reproducibility was evaluated by calculation of intraindividual coefficients of variation (CV) and intraclass correlation coefficients (ICC) for single measurements as well as for seven repeated measurements per study day.

Results

Image brightness, short-term reproducibility, and day-to-day reproducibility were significantly improved using ART of 100 frames compared to one and 16 frames. Short-term CV was reduced from 0.94?±?0.31 % and 0.91?±?0.54 % in scans of one and 16 frames to 0.56?±?0.42 % in scans of 100 averaged frames (P?≤?0.003 each). Day-to-day CV was reduced from 0.98?±?0.86 % and 0.78?±?0.56 % to 0.53?±?0.43 % (P?≤?0.022 each). The range of ICC was 0.94 to 0.99. Sample size calculations for detecting changes of RNFLT over time in the range of 2 to 5 μm were performed based on intraindividual variability.

Conclusion

Image quality and reproducibility of mean peripapillary RNFLT measurements using SD-OCT is improved by averaging OCT images with eye-tracking compared to unaveraged single frame images. Further improvement is achieved by increasing the amount of frames per measurement, and by averaging values of repeated measurements per session. These strategies may allow a more accurate evaluation of RNFLT reduction in clinical trials observing optic nerve degeneration.     

Spatial correlation of mouse photoreceptor-RPE thickness between SD-OCT and histology

Spectral Domain Optical Coherence Tomography (SD-OCT) applied to the mouse retina has been limited due to inherent movement artifacts and lack of resolution. Recently, SD-OCT scans from a commercially available imaging system have yielded retinal thickness values comparable to histology. However, these measurements are based on single point analysis of images. Here we report that using the Spectralis HRA + OCT Spectral Domain OCT and Fluorescein Angiography system (Heidelberg Engineering, Heidelberg, Germany), retinal thickness of linear expanses from SD-OCT data can be accurately assessed. This is possible by the development of a Spectralis-compatible ImageJ plug-in that imports 8-bit SLO and 32-bit OCT B-scan images, retaining scale and segmentation data and enabling analysis and 3D reconstruction. Moreover, mouse retinal layer thickness values obtained with this plug-in exhibit a high correlation to thickness measurements from histology of the same retinas. Thus, use of this ImageJ plug-in results in reliable quantification of long retinal expanses from in vivo SD-OCT images.     

Comparison of two different spectral domain optical coherence tomography devices in the detection of localized retinal nerve fiber layer defects

Purpose

To compare the detection of localized retinal nerve fiber layer (RNFL) defects by two different spectral domain optical coherence tomography (SD-OCT) devices.

Methods

Eyes of 42 normal control subjects and 48 patients with a localized RNFL defect on red-free fundus photographs were imaged by the Cirrus (Carl Zeiss Meditec, Dublin, CA, USA) and 3D OCT (Topcon, Tokyo, Japan) devices. We compared sensitivities, specificities, and area under the receiver operating characteristic curves (AUCs) of circumpapillary RNFL (cpRNFL) thickness and ganglion cell complex (GCC) parameters between the two devices.

Results

The devices provided different cpRNFL thickness measurements. The highest sensitivities at fixed specificities of 80 % (Cirrus: 83.3 %; 3D OCT: 77.1 %) and 95 % (Cirrus: 69.8 %; 3D OCT: 68.8 %) and the largest AUCs (Cirrus: 0.90; 3D OCT: 0.88) obtained by the cpRNFL parameters of the two devices were similar. Based on the internal normative database, the deviation-from-normal map of the Cirrus OCT device and the 36-segment map of the 3D OCT device had the highest sensitivity (89.6 and 91.7 %, respectively). Among the macular GCC parameters of the 3D OCT device, inferior macular RNFL thickness had the highest sensitivity (81.2 % at a specificity of 80 %) and the largest AUC (0.89).

Conclusions

Although the two SD-OCT devices have different measurement protocols, they showed similar abilities for the detection of a localized RNFL defect.     

Reproduzierbarkeit der Messung der peripapillären Nervenfaserschichtdicke

Purpose

The aim of this study was to compare the intra- and inter-examiner reproducibility of measurements obtained by optical coherence tomography (OCT) and retinal thickness analyzer (RTA).

Patients and methods

During a period of 2 months, 22 eyes of 16 patients and 6 healthy subjects were included. Two examiners (EMH, RK) successively performed three measurements of the peripapillary retinal nerve fibre layer (RNFL) thickness with RTA and OCT. The reproducibility of three individual measurements of one examiner (intra-examiner) as well as the reproducibility of the measurements between both examiners (inter-examiner) was evaluated using the Friedman test and sign test.

Results

The average thickness of the peripapillary RNFL was 154.4 µm for the first investigator (EMH) and 155.1 µm for the other investigator (RK) measured with RTA. The results obtained by OCT were 137.3 µm (EMH) and 138.9 µm (RK), respectively, generally indicating a threefold smaller range. Comparing the three measurements of one single examiner, no appreciable intra-observer dependency neither for RTA (EMH: p=0.19, RK: p=0.95) nor for OCT (EMH: p=0.51, RK: p=0.62) was observed. Inter-examiner analysis for RTA and OCT also revealed an acceptable reproducibility.

Conclusions

Measurements of peripapillary RNFL thickness using RTA and OCT exhibited intra- and inter-observer agreement.     

Biometrie des vorderen Augensegmentes mittels optischer Kohärenztomographie

Background

Digital visualisation of the anterior eye segment is becoming more and more important. Two commercially available optical coherence tomographs (OCTs) — Visante OCT (Carl-Zeiss Meditec Jena) and Slit Lamp-OCT (SL-OCT. Heidelberg Engineering) — were evaluated. Additionally, a manual and a semiautomatic analysis mode for the anterior segment biometry were compared using the SL-OCT.

Patients and methods

Fifty eyes were examined by three ophthalmologists with complete eye status in a standardised fashion. Three anterior segment scans (horizontal and vertical) were performed using the Visante OCT and the SL-OCT. The manual integrated analysis mode of the Visante OCT was used. The best centred SL-OCT scan was analysed with a manual (EyelabGlobal/4Optics) and a semiautomatic procedure (HEYEX). Central corneal thickness (CCT), anterior chamber depth (ACD), pupillary diameter (PD) and chamber angle–angle distance (CAAD) were compared.

Results

Comparison of Visante OCT and SL-OCT: The horizontal CAAD was 11.65±0.47 mm for the Visante-OCT and 12.0±0.57 mm for the SL-OCT (p=0,002), vertical scans were comparable in 10 eyes only. The CCT was 568±61 µm with the Visante-OCT and 581±48 µm with the SL-OCT (n=35, p=0.03) in horizontal scans and 565±62 µm with the Visante-OCT and 568±51 µm with the SL-OCT in vertical scans (n=27, not significant). Deviation of the two measurement methods varied between –54.7 and 80.8 µm in horizontal scans and between –84.9 and 91.1 µm in vertical scans from the mean difference of the two methods. Comparison of the analysis programs: Horizontal scans: Manual analysis correlated with semiautomatic analysis in CCT as 581±51 µm vs. 572±53 µm (r=0.903), ACD 2.89±0.74 mm vs. 2.08±0.72 mm (r=0.98), PD 5.22±2.12 mm vs. 5.14±1.91 mm (r=0.917) and CAAD 11.59±1.02 mm vs. 11.79±0.6 mm (r=0.47). The manual and a semiautomatic analysis modes for CCT and ACD differed significantly (p<0.026). Vertical scan: Manual versus semiautomatic analysis modes correlated in CCT as 578±65 µm vs. 573±63 µm (r=0.593), ACD with 3.04±0.83 mm vs. 3.03±0.75 mm (r=0.92), PD with 5.28±1.99 mm vs. 5.45±2.00 mm (r=0.899) and CAAD with 11.75±0.66 mm vs. 11.82±0.6 mm (r=0.537).

Conclusions

A near complete investigation of the cornea–anterior–chamber–iris segment is provided by the horizontal scan compared with the vertical scan. The noncontact OCT method is an easily handled tool for visualisation and biometry of the anterior eye segment. The investigated OCTs show a limited comparability. A complete analysis was possible in all eyes with the EyelabGlobal system in contrast to the HEYEX analysis software. Both analysis programs provided comparable measurements of the anterior eye segment. The semiautomatic biometrical mode may reduce the analysis time in qualitative excellent and well-centred scans to 10%.     

Corneal thickness mapping by 3D swept‐source anterior segment optical coherence tomography

Purpose: To assess accuracy and repeatability of central corneal thickness (CCT) measurements obtained by swept‐source anterior segment optical coherence tomography (AS‐OCT), spectral‐domain retinal OCT with corneal module and ultrasound pachymetry (USP), and to assess repeatability of pachymetric mapping with AS‐OCT. Methods: 50 healthy volunteers were recruited. A single, experienced operator analysed the right eye of each participant twice in the same session with AS‐OCT (‘corneal map’ routine), retinal OCT and USP. CCT measurements were compared using repeated‐measures analysis of variance, Bonferroni test, Pearson correlation and Bland‐Altman plots. Repeatability of thickness maps and CCT measurements were assessed using Alpha of Cronbach, intraclass correlation coefficient (ICC) and coefficient of repeatability. Results: Mean CCT ± SD was 540 ± 28.9 μm for AS‐OCT, 544 ± 29.5 μm for retinal OCT and 549.3 ± 31.7 μm for USP; the differences were statistically significant (p < 0.01). CCT measurements obtained with the three instruments were highly correlated: r was 0.965 for AS‐OCT/USP, 0.962 for retinal OCT/USP and 0.984 for AS‐OCT/retinal OCT comparison. The repeatability of CCT measurements was higher for AS‐OCT than for the other devices (p < 0.001). Repeatability of pachymetric maps was excellent (ICC = 0.999). Conclusions: Pachymetric maps by swept‐source AS‐OCT showed excellent repeatability. CCT measurements obtained by AS‐OCT, USP and retinal OCT were highly correlated although not identical.     

SD-OCT检测视网膜神经纤维层厚度的可重复性研究

目的 探讨频域光学相干断层成像术(SpectralDomain Opticalcoherence tomography,SD-OCT)检测正常人视网膜神经纤维层(RNFL)厚度的可重复性结果.方法 同一操作者应用SD-OCT的6mx6m3D视乳头200X200扫描模式(包括200次B扫描和200次A扫描)对36例(43只眼)正常人进行视乳头环形扫描,测量全周平均厚度、4个象限和12钟点位的RNFL厚度值,采用变异系数(coefficient of variation,CV)和类内相关系数(intraclasscorrelationcoemcient,ICC)来计算.结果 全周视网膜神经纤维层厚度的ICC和CV值范围分别为0.853～0.991,1.4%～5.8%.其中,上方象限的ICC值最高为0.991.鼻侧象限的ICC值最低为0.853,全周平均厚度的CV最低为1.4%,3钟点位的CV最高为5.8%.ICC值分别是上方>平均>颞侧>下方>鼻侧.与其相对应的12钟点位,11点、12点和1点的ICC值分别为0.974、0.988、0.964,2点、3点、4点的ICC值分别为0.940、0.853、0.907,5点、6点、7点的ICC值分别为0.933、0.959、0.980,8点、9点、10点的ICC值分别为0.965、0.924、0.966.结论 SD-OCT检测正常人视网膜神经纤维层厚度可重复性几乎一致,在检测青光眼的早期进展方面更有用.     

SD-OCT检测视网膜神经纤维层厚度的可重复性研究

目的 探讨频域光学相干断层成像术(SpectralDomain Opticalcoherence tomography,SD-OCT)检测正常人视网膜神经纤维层(RNFL)厚度的可重复性结果.方法 同一操作者应用SD-OCT的6mx6m3D视乳头200X200扫描模式(包括200次B扫描和200次A扫描)对36例(43只眼)正常人进行视乳头环形扫描,测量全周平均厚度、4个象限和12钟点位的RNFL厚度值,采用变异系数(coefficient of variation,CV)和类内相关系数(intraclasscorrelationcoemcient,ICC)来计算.结果 全周视网膜神经纤维层厚度的ICC和CV值范围分别为0.853～0.991,1.4%～5.8%.其中,上方象限的ICC值最高为0.991.鼻侧象限的ICC值最低为0.853,全周平均厚度的CV最低为1.4%,3钟点位的CV最高为5.8%.ICC值分别是上方>平均>颞侧>下方>鼻侧.与其相对应的12钟点位,11点、12点和1点的ICC值分别为0.974、0.988、0.964,2点、3点、4点的ICC值分别为0.940、0.853、0.907,5点、6点、7点的ICC值分别为0.933、0.959、0.980,8点、9点、10点的ICC值分别为0.965、0.924、0.966.结论 SD-OCT检测正常人视网膜神经纤维层厚度可重复性几乎一致,在检测青光眼的早期进展方面更有用.     

SD-OCT检测视网膜神经纤维层厚度的可重复性研究

目的 探讨频域光学相干断层成像术(SpectralDomain Opticalcoherence tomography,SD-OCT)检测正常人视网膜神经纤维层(RNFL)厚度的可重复性结果.方法 同一操作者应用SD-OCT的6mx6m3D视乳头200X200扫描模式(包括200次B扫描和200次A扫描)对36例(43只眼)正常人进行视乳头环形扫描,测量全周平均厚度、4个象限和12钟点位的RNFL厚度值,采用变异系数(coefficient of variation,CV)和类内相关系数(intraclasscorrelationcoemcient,ICC)来计算.结果 全周视网膜神经纤维层厚度的ICC和CV值范围分别为0.853～0.991,1.4%～5.8%.其中,上方象限的ICC值最高为0.991.鼻侧象限的ICC值最低为0.853,全周平均厚度的CV最低为1.4%,3钟点位的CV最高为5.8%.ICC值分别是上方>平均>颞侧>下方>鼻侧.与其相对应的12钟点位,11点、12点和1点的ICC值分别为0.974、0.988、0.964,2点、3点、4点的ICC值分别为0.940、0.853、0.907,5点、6点、7点的ICC值分别为0.933、0.959、0.980,8点、9点、10点的ICC值分别为0.965、0.924、0.966.结论 SD-OCT检测正常人视网膜神经纤维层厚度可重复性几乎一致,在检测青光眼的早期进展方面更有用.     

Effect of OCT volume scan density on thickness measurements in diabetic macular edema

Purpose

To evaluate the impact of reducing B-scan frame-sampling density on retinal thickness measurements using spectral domain optical coherence tomography (SD-OCT) in eyes with diabetic macular edema (DME).

Methods

We retrospectively collected OCT data for 64 eyes of 43 patients undergoing imaging for DME using the Cirrus HD-OCT 512 × 128 macular cube protocol. For each case, raw OCT data were imported into the 3D-OCTOR software, and retinal thickness maps were generated using all 128 B-scans and for lower densities of B-scans ranging from every other scan to only four scans (every 30-s B-scan). Maps were generated before and after manual correction of retinal boundary segmentation errors. The foveal central subfield (FCS) and total macular volume (TMV) values were used to compare thickness maps of varying densities.

Results

The mean difference in FCS retinal thickness and TMV increased as the B-scan density was reduced, particularly when the density was reduced to fewer than 16 B-scans over 6 mm. At a density of 16 B-scans, the mean absolute difference in FCS thickness was 2.43 μm (0.79%), with a maximum of 10.1 μm (4.09%). At this density, the mean difference in TMV was 0.012 mm³ (0.13%), with a maximum difference of 0.04 mm³ (0.47%). Manual correction of OCT segmentation errors resulted in a difference in FCS thickness of ≥10 μm in only 12.5% of cases, with a maximum difference of 115.7 μm.

Conclusion

A minimum of 16 equally spaced B-scans appear necessary to generate retinal thickness measurements similar to those produced using all 128 B-scans in eyes with DME. Manual correction of segmentation errors appeared to have a clinically meaningful effect in a small minority of cases. These results may have implications for the design of SD-OCT imaging and grading protocols in clinical trials of DME, particularly when using multiple SD-OCT instruments that acquire varying numbers of B-scans.     

A Novel Technique of Adjusting Segmentation Boundary Layers to Achieve Comparability of Retinal Thickness and Volumes between Spectral Domain and Time Domain Optical Coherence Tomography

Purpose. The quantitative assessment of retinal thickness and volume varies according to the optical coherence tomography (OCT) machine used due to differences in segmentation lines. We describe a novel method of adjusting the segmentation lines of spectral-domain OCT (SD-OCT) to enable comparison with time-domain OCT (TD-OCT), and assess factors affecting its accuracy. Methods. In a prospective study, SD-OCT (Spectralis OCT) and TD-OCT (Stratus OCT) were sequentially performed on 200 eyes of 100 healthy individuals. Central retinal thickness (CRT), central point thickness (CPT), and 1-mm volume of the Early Treatment Diabetic Retinopathy Study grid were compared between the two machines. The segmentation lines on SD-OCT were manually adjusted by a trained operator and the parameters compared again with TD-OCT. Results. The mean CRTs of Spectralis and Stratus were significantly different (268.2 μm vs. 193.9 μm, P < 0.001). After adjustment of segmentation lines, the mean adjusted Spectralis CRT was 197.3 μm, with the difference between SD-OCT and TD-OCT measurements decreasing from 74.3 μm to 3.4 μm (P < 0.001). The difference between the adjusted Spectralis and Stratus CRTs was smallest for high myopes (≤ -6.0 diopters [D]) compared with those with moderate and low myopia (1.5 μm vs. 3.5 μm and 4.6 μm, respectively; P < 0.001). Similar trends were obtained for central 1-mm volumes and CPT. Interoperator and intraoperator repeatability for adjustment of the segmentation lines were good, with an intraclass correlation of 0.99 for both. Conclusions. Manual adjustment of SD-OCT segmentation lines reliably achieves retinal thickness and volume measurements that are comparable to that of TD-OCT. This is valuable to allow comparisons in multicenter clinical trials where different OCT machines may be used.     

Comparison of Central Corneal Thickness with Ultrasound Pachymetry,Noncontact Specular Microscopy and Spectral Domain Optical Coherence Tomography

Background: To determine the agreement of central corneal thickness (CCT) measurements taken with ultrasonic pachymetry (USP), spectral domain optical coherence tomography (SD-OCT) and noncontact specular microscopy (NSM).

Methods: A prospective, observational, cross-sectional study was performed in the outpatient ophthalmology clinic. CCT was measured in a total of 147 eyes of 147 consecutive healthy patients with USP, NSM, and SD-OCT. Same examiner performed all examinations. Bland–Altman plots were used to evaluate the agreement between instruments.

Results: The average CCT values obtained by USP, NSM, and SD-OCT were 555 ± 37 µm, 554 ± 34 µm, and 546 ± 34 µm, respectively. There was a strong correlation between instruments: USP with SD-OCT (r = 0.937, p < 0.01), USP with NSM (r = 0.943, p < 0.01) SD-OCT with NSM (r = 0.975, p < 0.01) for CCT. The mean differences (lower/upper limit of agreement) for CCT measurements were ?10 ± 12.9 µm (15.28/?35.28) between SD-OCT and USP, ?8.1 ± 7.7 µm (7/?23.2) between SD-OCT and NSM, and 1.8 ± 12.3 µm (25.9/?22.3) between USP and NSM.

Conclusions: USP and NSM were found to have comparable CCT measurements and these two methods can be used correspondingly. However, CCT measurements by SD-OCT were lower when compared to other methods.     

Reproducibility of retinal nerve fiber thickness measurements using the stratus OCT in normal and glaucomatous eyes

PURPOSE: To determine the reproducibility of Stratus Optical Coherence Tomography (OCT) retinal nerve fiber layer (RNFL) measurements around the optic nerve in normal and glaucomatous eyes. METHODS: One eye was chosen at random from 88 normal subjects and 59 glaucomatous subjects distributed among mild, moderate, and severe glaucoma, determined by visual field testing. Subjects underwent six RNFL thickness measurements performed by a single operator over a 30-minute period with a brief rest between sessions. Three scans were taken with the high-density Standard RNFL protocol, and three were taken with the Fast RNFL protocol, alternating between scan protocols. RESULTS: Reliability, as measured by intraclass correlation coefficient (ICC), was calculated for the overall mean RNFL thickness and for each quadrant. The ICC for the mean Standard RNFL thickness (and lower 95% confidence interval [CI]) in normal and glaucomatous eyes was 0.97 (0.96 CI) and 0.98 (0.97 CI), respectively. The ICC for the mean Fast RNFL thickness in normal and glaucomatous eyes was 0.95 (0.93 CI) and 0.97 (0.95 CI), respectively. Quadrant ICCs ranged between 0.79 and 0.97, with the nasal quadrant being the least reproducible of all four quadrants, using either the Standard or Fast RNFL program. The test-retest variability ranged from 3.5 microm for the average RNFL thickness measurements in normal eyes to 13.8 microm for the nasal quadrant measurements in glaucomatous eyes, which appeared to be the most variable. CONCLUSIONS: Reproducibility of RNFL measurements using the Stratus OCT is excellent in normal and glaucomatous eyes. The nasal quadrant appears to be the most variable measurement. Standard RNFL and Fast RNFL scans are equally reproducible and yield comparable measurements. These findings have implications for the diagnosis of glaucoma and glaucomatous progression.     

正常人眼视网膜神经纤维层的OCT光学特征定量测量的可重复性

目的评估使用光学相干断层扫描（OCT）定量测量视网膜神经纤维层（RNFL）光学特征的可重复性。方法横断面研究。共连续纳入正常受检者50例（50眼）,应用Cirrus OCT的5线高清扫描模式和视盘数据方模式,在1 d内的不同时间获取RNFL的原始图像数据,并通过2位检查者在双盲情况下使用自编程图像处理软件分析RNFL的光学特征,包括吸光度和吸光系数。运用组内标准差,组内相关系数（ICC）,变异系数（COVs）和Bland-Altman方法来评估OCT光学特征定量测量和自编程图像分析软件的可重复性。结果受检者的年龄为（51.2±8.1）岁。黄斑区/视盘周边区RNFL厚度、吸光度和吸光系数分别为（34.38±3.45）µm/（89.07±3.19）µm、（117.90±6.02）灰度/（138.10±18.25）灰度和（1.99±0.16）/（3.48±0.64）。RNFL厚度、吸光度以及吸光系数的ICC为0.712～0.952,COVs为2.20%～6.54%。黄斑区/视盘周边区RNFL吸光度OCT 2次测量结果之间的平均偏差分别为-0.01灰度/0.33灰度,吸光系数为0.01/-0.01。自编程图像处理软件测量的RNFL厚度、吸光度及吸光系数的ICC为0.928~0.979,COVs均小于6%（1.35%~4.24%）。结论OCT光学特征定量测量和自编程图像分析软件定量测量RNFL光学特征可取得较高的可重复性。     

Quantitative thickness measurement of retinal layers imaged by optical coherence tomography

PURPOSE: To report an image analysis algorithm that was developed to provide quantitative thickness measurement of retinal layers on optical coherence tomography (OCT) images. DESIGN: Prospective cross-sectional study. METHODS: Imaging was performed with an OCT3 commercial instrument in 10 visually normal healthy subjects. A dedicated software algorithm was developed to process the raw OCT images and detect the depth location of peaks from intensity profiles. Quantitative thickness measurements of three retinal layers, in addition to total retinal thickness, were derived. Total retinal thickness measurements obtained by the algorithm were compared with measurements provided by the standard OCT3 software. RESULTS: The total retinal thickness profile demonstrated foveal depression, corresponding to normal anatomy, with a thickness range of 160 to 291 microm. Retinal thickness measured by the algorithm and by the standard OCT3 software were highly correlated (R = 0.98). The inner retinal thickness profile predictably demonstrated a minimum thickness at the fovea, ranging between 58 to 217 microm along the 6-mm scan. The outer retinal thickness profile displayed a maximum thickness at the fovea, ranging between 66 to 107 microm along the 6-mm scan. The photoreceptor outer segment thickness profile was relatively constant along the 6-mm scan through the fovea, ranging between 42 to 50 microm. The intrasubject variabilities of the inner retina, outer retina, and photoreceptor outer segment thickness was 14, 10, and 6 microm, respectively. CONCLUSIONS: Thickness measurements of retinal layers derived from OCT images have potential value for objectively documenting disease-related retinal thickness abnormalities and monitoring progressive changes over time.     

Influence of Anterior Segment Power on the Scan Path and RNFL Thickness Using SD-OCT

Purpose. Retinal nerve fiber layer (RNFL) thickness measures with spectral domain-optical coherence tomography (SD-OCT) provide important information on the health of the optic nerve. As with most retinal imaging technologies, ocular magnification characteristics of the eye must be considered for accurate analysis. While effects of axial length have been reported, the effects of anterior segment optical power on RNFL thickness measures have not been described fully to our knowledge. The purpose of our study was to determine the influence of the optical power change at the anterior corneal surface, using contact lenses, on the location of the scan path and measurements of RNFL thickness in normal healthy eyes. Methods. We recruited 15 normal subjects with less than 6 diopters (D) of ametropia and no ocular pathology. One eye of each subject was selected randomly for scanning. Baseline SD-OCT scans included raster cubes centered on the optic nerve and macula, and a standard 12-degree diameter RNFL scan. Standard 12-degree RNFL scans were repeated with 10 separate contact lenses, (Proclear daily, Omafilcon A/60%) ranging from +8 to -12 D in 2-D steps. The extent of the retinal scan, and RNFL thickness and area measures were quantified using custom MATLAB programs that included ocular biometry measures (IOL Master). Results. RNFL thickness decreased (0.52 μm/D, r = -0.33, P < 0.01) and the retinal region scanned increased (0.52%/D, r = 0.97, P < 0.01) with increase in contact lens power (-12 to +8). The normalized/percentage rates of change of RNFL thickness (-0.11/mm, r = -0.67, P < 0.01) and image size (0.11/mm, r = 0.96, P < 0.01) were related to axial length. Changes in the retinal region scanned were in agreement with transverse scaling, computed with a three surface schematic eye (R(2) = 0.97, P < 0.01). RNFL area measures, that incorporated the computed transverse scaling, were not related significantly to contact lens power (863 μm(2)/D, r = 0.06, P = 0.47). Conclusions. Measurements of RNFL thickness by SD-OCT are dependent on the optics of the eye, including anterior segment power and axial length. The relationships between RNFL thickness measures and optical power are a direct reflection of scan path location with respect to the optic nerve head rim, caused by relative magnification. An incorporation of transverse scaling to RNFL area measures, based on individualized ocular biometry, eliminated the magnification effect.     

Retinal nerve fibre layer thickness profile in normal eyes using third-generation optical coherence tomography

AIMS: To establish four normal retinal nerve fibre layer (RNFL) thickness radial profiles based on third-generation optical coherence tomography (OCT) and to compare them with previously reported histologic measurements. METHODS: A total of 20 normal eyes were studied. A circular scan was adjusted to the size of the optic disc and three scans were performed with this radius and every 200 microm thereafter, up to a distance of 1400 microm. Four different radial sections (superotemporal, superonasal, inferonasal, and inferotemporal) were studied to establish RNFL thickness OCT profiles. Additionally, two radial scans orientated at 45 and 135 degrees crossing the optic disc centre were performed in six of 20 eyes, and RNFL thickness was measured at disc margin. RESULTS: Quadrant location and distance from disc margin interaction in RNFL thickness was statistically significant (P<0.001). The RNFL thickness decreased (P<0.001) as the distance from the disc margin increased for all sections. The measurements automatically generated by the OCT built-in software were thinner (P<0.001) than histologic ones close to the disc margin. CONCLUSIONS: Four normal OCT RNFL profiles were established and compared with histological data obtained from the same area. RNFL measurements assessed by OCT 3 were significantly thinner close to the optic disc margin.     

Optic Disc and Macular Imaging in Blind Eyes from Non-glaucomatous Optic Neuropathy: A Study with Spectral-domain Optical Coherence Tomography

The purpose of this study was to determine and compare the optic disc and macular thickness measurements using two spectral-domain optical coherence tomography (SD-OCT) instruments in long-standing blind eyes diagnosed with non-glaucomatous optic neuropathies (NGON). A prospective observational case-series design was used. Twelve eyes from 12 NGON patients with no light perception for at least 6 months underwent optic disc and macular imaging with Cirrus HD-OCT and Spectralis OCT. The correlation between the peripapillary retinal nerve fibre layer (PRNFL) and macular ganglion cell layer and inner plexiform layer (GCL+IPL) thicknesses, and between the duration of no light perception (NLP) and PRNFL/GCL+IPL thicknesses were determined using Spearman’s correlation analysis. The mean average PRNFL thickness was 55.9 ± 4.8 µm for Cirrus HD-OCT, which was significantly thicker than that measured by Spectralis OCT (31.9 ± 7.4 µm; p < 0.001). The mean central macular thickness on Cirrus HD-OCT was normal, but there was global thinning at the other macular areas. The mean average GCL+IPL thickness on Cirrus HD-OCT was 51.8 ± 5.8 µm. There was a good correlation between average PRNFL thickness and GCL+IPL thickness (r = 0.830, p = 0.002); however, there was no significant correlation between the duration of NLP to the average PRNFL thickness (on either instruments) or GCL+IPL thickness on Cirrus HD-OCT (p > 0.7). These results show that there was residual PRNFL thickness in NGON eyes with NLP, which varied significantly between SD-OCT instruments. The values of the residual PRNFL and GCL+IPL thicknesses in blind eyes (the “floor” effect) may be useful for prognostic purposes for patients with partial optic atrophy.